Digital input modules, used in industrial and building automation, can contain anywhere from as few as four to as many as thousands of digital inputs or “channels” for interfacing with various sensors and switches. These modules are typically used in conjunction with a specialized microcomputer referred to as a Programmable Logic Controller or PLC. With a typical digital input module of the prior art, each input, when in the “on” state, can draw, for example, 100 to 300 mW of power from the module's power supply. This high power usage limits the density (and therefore the maximum number) of channels that can be provided by digital input modules of the prior art due to design requirements including heat dissipation and maximum power consumption.
A digital input module is primarily responsible for the detection of binary (on/off) signals from digital sensors such as proximity sensors, level sensors, limit switches, push buttons, etc., that operate off of a high voltage or “field power” supply. A field power supply is nominally about 24 volts DC (VDC) with a 30 VDC maximum rating. The digital input module converts the high voltage sensor signals to lower voltage signals (e.g. 5 VDC) that can be processed by the PLC. A digital input module also typically filters or de-bounces the sensor signals and may provide galvanic isolation of the high voltage sensors from the low voltage PLC for the safety of human operators as well as reliable operation of the low voltage controller circuits.
It is important to note that, in most cases, the detection of a valid logic “1” or logic “0” from industrial digital sensors requires measuring both the voltage from the sensor and the current flowing through it. For a valid logic “1”, it is required that both voltage and current be above certain preset thresholds. These thresholds are typically governed by IEC 61131-2 specifications as set forth by the International Electrotechnical Commission having a central office in Geneva, Switzerland, incorporated herein by reference.
There are several methods used to detect the high voltage (e.g. 24 VDC) binary output of industrial sensors and switches. One method is to use a discrete resistor divider circuit as illustrated in FIG. 1A. With this circuit, a resistor divider is used to detect a predefined voltage threshold and turn on an optocoupler once a voltage threshold and a current threshold are exceeded. The optocoupler serves as a galvanic isolator as well as a voltage level translator to supply a 5V binary signal, representing the state of the sensor, to a PLC.
In FIG. 1A, a typical resistor divider circuit 10 includes an input resistor 12, an input capacitor 14, a first divider resistor 16, a filter capacitor 18, a second divider resistor 20, a first current limiting resistor 22, an indicator LED 24, a second current limiting resistor 25, an optocoupler (or “opto-isolator” or “optical isolator”) 26, and a pull-up resistor 28. A sensor 30, represented here as a switch, connects the input resistor 12 to the field power supply (e.g. 24 VDC). The voltage applied to the optocoupler 26 is divided by the first divider resistor and, primarily, by the second divider resistor 20, although system designers would also have to account for other resistances that are in parallel with the second divider resistor 20. The capacitors 14 and 18 form a part of an RC filter which helps debounce the input signal developed by the activation of the sensor 30, and the optocoupler 26 provides galvanic isolation between the high-voltage side of the resistor divider circuit 10 (e.g. 24 VDC peak amplitude) and the low-voltage side of the resistor divider circuit 10 (e.g. 10 VDC peak amplitude).
The resister divider circuit 10 has the advantage of economy as it is fairly inexpensive to implement. A problem with resistor divider circuit 10 is that it has high power consumption due to current continuously flowing through the divider resistors 16 and 20 when the sensor 30 is activated. In fact, with the resistor divider circuit 10, power consumption as set forth by the equation P=V*I increases quadratically with the sensor voltage, since the current increases linearly according to Ohm's law: I=V/R. Power consumption of the resistor divider circuit with the sensor 30 activated (e.g. the switch is closed) is therefore P=Vs2/R, where Vs is the field voltage.
As illustrated in FIG. 1B, another method utilizes a current limiter circuit 32 including an input resistor 34, an input capacitor 36, a current limiter 38, a voltage/current comparator 40, a low pass filter 42 and a clock (or “oscillator”) 44. A sensor 46, again represented here by a switch, couples the input resistor 34 to a 24 VDC field power supply.
In operation, the current limiter circuit 32 allows the current to rise linearly with voltage provided by sensor 46 up to a detection threshold and is then clamped from rising any further by the current limiter 38. No further increases in input current are possible as the sensor voltage rises to its maximum level set by the field power supply. Detector 40 is used to detect the binary state of the sensor and convert it down to 5V. Digital low pass filter 42, which is clocked by clock 44, is then used after the detector 40 to filter and de-bounce the sensor signal.
The current limiter circuit 32 consumes less power than the resistor divider circuit 10 because the current limiter 38 limits the maximum current that can be drawn from the high voltage sensor 46. However, the current limiter continues to draw current at the current limit (“clamp”) level iL as long the sensor 46 output is at a high voltage level. Therefore, while the power consumption of the current limiter circuit of FIG. 1B is less than the power consumption of the resistor divider circuit of FIG. 1A, the current limiter circuit still consumes a considerable amount of power which can limit channel density and which necessitates larger power supplies.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.